In the electrophotographic or xerographic art it is customary to utilize photoreceptor plates having at least an external photoconductive insulating layer and a charge conductive supporting substrate. Generally, a photoconductive layer is uniformly electrostatically charged in the absence of light or other activating radiation and, thereafter, exposed to a light pattern which can correspond to a negative image. The areas of the photoconductive layer which are so exposed selectively lose their charge much more rapidly than non-exposed areas. As a result, the photoconductive layer at least temporarily retains a charge corresponding essentially to a latent positive image. This image can then be conveniently developed to form a visible positive image by contacting with oppositely charged pigmented particles commonly identified as toner particles which will adhere mostly to the charged areas. The resulting image may optionally be permanently affixed to the photoconductor if the imaging layer is not to be reused. This usually occurs with binder-type photoconductive film where the photoconductive imaging layer is also an integral part of the finished copy.
Where "plain paper" copying systems are involved, however, the latent image is conveniently developed on the imaging surface of a reusable photoconductor, or transferred to another surface such as a sheet of paper, and thereafter developed. After a latent image is developed on the imaging surface of a reusable-type photoconductor, it is transferred to another substrate and then permanently affixed by using any one of a variety of well-known techniques such as by overcoating with a transparent film, or by thermal fusion of the toner particles to the sheet. In such a copying system the materials in the photoconductive layer must be capable of rapidly changing from an insulative, to a charge-conductive, and then back to an insulative condition to permit cyclic use of the imaging surface. Failure to revert back to the insulative state before each succeeding charging sequence will result in a high dark decay rate commonly referred to as "fatigue". In the past, the problem has been controlled, to some extent, simply by selection of those photoconductive materials having the best known rapid switching capacity. Typical of such materials are anthracene, poly(N-vinylcarbazole), sulfur, selenium, selenium alloys, metal-free phthalocyanines, etc., and mixtures thereof (U.S. Pat. No. 2,297,691).
While organic photoconductive materials such as poly(N-vinylcarbazole) generally have good dark decay characteristics, they tend to lack sufficient inherent photosensitivity to be completely competitive with selenium. For this reason, they are usually used together with "activators". Poly(vinylcarbazoles), for example, are sensitized with 2,4,7-trinitro-9-fluorenone to obtain improved photoresponse, discharge characteristics (ref. U.S. Pat. No. 3,484,237). There are also other organic resins which are traditionally considered non-photoconductive, but which can be sensitized with Lewis Acids to form charge-transfer complexes which are photoresponsive at the visible end of the spectrum. U.S. Pat. Nos. 3,408,181; 3,408,182; 3,408,183; 3,408,184; 3,408,185; 3,408,186; 3,408,187; 3,408,188; 3,408,189; and 3,408,190 are of interest in this area.
For all practical purposes, the amount of sensitization of both photoconductive and non-photoconductive resins depends upon the concentration of the activator; within limits, the higher the loading, the greater the photoresponse obtained. Unfortunately, however, loadings exceeding about 10 weight percent of the photoconductive composition will usually tend to impair mechanical and/or other photoconductive properties of the sensitized composition. Excessive amounts of activator in either a photoconductive or a nonphotoconductive material of the type disclosed in the above patents will tend to crystallize out of the photoconductive composition.
The above inherent limitations make it very difficult and often times impossible to obtain the much-desired marriage of a high quantum efficiency photoconductor with a tough, transparent, active matrix material having a low injection threshold.
One very useful discovery directed to the above goal utilizes various types of protective polymeric overcoats capable of holding a charge of high field strength on an external surface while also permitting selective transmittal of holes (i.e. charge transfer) from the photoconductive or generating layer to obtain an electrostatic image.
While the last mentioned system offers many substantial advantages, there still remains the need for further improvement, particularly in the area of image resolution and speed, without causing a corresponding increase in the rate of dark discharge.
It is believed that xerographic plates having improved speed and resolution can be obtained if the plates are made of a juxtaposed layer of two materials, one material facilitating photogeneration while the other facilitates charge transport. Except for the situation in which photogeneration is effected from impurity centers, greater sensitivity (speed) is correlated with narrowed band gaps. Unfortunately, however, a reduction in band gap generally also results in substantially greater thermal generation and corresponding faster rate of dark decay.